This invention relates generally to EDM (Electrical Discharge Machining) apparatus and is specifically concerned with a new and improved electronic depth control for such apparatus.
EDM has been successfully applied to machining of precision holes in objects. The advantages of EDM are well-known in this respect and will not be repeated here. Suffice it to say that EDM can achieve precision results where other procedures and apparatus would be totally incapable of even approximating the performance of EDM. In spite of the manifold advantages of EDM, there are particular products where extreme precision in machining holes is required. An example of one such situation is the machining of multiple blind holes in a workpiece where the hole depths are critical. Another example is where multiple through-holes are being machined, and it is important that all electrodes break through at substantially the same time.
Advances in EDM control apparatus are demonstrated by assignee's control system identified by the designation CP-60. The CP-60 control system is a microprocessor-based control which greatly enhances the performance of EDM apparatus. That system represents an advance toward the ultimate objective of a completely adaptive control system. In an adaptive control system actual machining conditions, or indicators of actual machining conditions, are monitored while in progress, and the manner of control is concurrently adapted in respect of the monitored conditions or indicia thereof so that optimum machining action occurs. By way of example an adaptive control may on the one hand provide a more rapid machining procedure where the actual machining is easier than anticipated, and it may on the other hand provide a slower machining rate where the actual machining operation is more difficult than originally anticipated.
The present invention constitutes an improvement upon an EDM control system toward the ultimate objective of a completely adaptive control system. The present invention in this regard, is directed to an electronic depth controller which is employed in operative association with the main EDM control. The electronic depth controller of the present invention acts upon selected input data relating to advance of the machining electrodes, and hence depth of holes machined in the workpiece, and interacts with the main control system in a manner whereby difficult hole machining problems, such as those referred to above in connection with blind holes and simultaneous breakthrough of multiple holes, are solved. Accordingly the present invention is particularly adapted for enhancing the capability of EDM apparatus in accurately machining multiple blind holes to precision depths and also to accomplishing simultaneous breakthrough where multiple through-holes are being machined in a workpiece. The basic function of the electronic depth controller of the present invention will therefore be recognized as assuring precise depth location of the tips of the EDM electrodes. This can consequently eliminate opposite wall damage when machining into a cavity and eliminate scarfing. By virtue of the electronic character of the depth controller of the present invention, in conjunction with the electronic character of the main control system, virtually instantaneous response to actual machining condition occurs whereby depth control and simultaneous breakthrough are accomplished with precision.
Briefly, the preferred embodiment of depth controller comprises its own microprocessor which receives selectable input data from input selector switches which are mounted on a control panel. The microprocessor acts upon the data supplied by the selectable input switches and provides output data based upon the selectable input data which is conveyed to the microprocessor of the main control. The electronic depth controller thereby provides information relating to machining depths and the main control acts upon this information in controlling the machining process. The input data which is supplied by the control panel switches comprises an initial depth limit, a first desired machining increment, a second desired machining increment, and the number of desired machining increments. Furthermore, the control panel contains a selector switch for selecting a desired mode of operation of the depth controller.
The microprocessor of the depth controller is programmed with separate sets of operating instructions and the particular set of instructions which is executed is determined by the mode selector switch on the control panel. In a first mode of operation the microprocessor acts upon the selected initial depth limit, the selected desired first machining increment and the desired number of machining increments whereby the microprocessor of the electronic depth controller provides signals to the main microprocessor control whereby the electrodes are advanced in unison initially to the desired initial depth limit, and are thereafter advanced in a succession of machining increments with each machining increment corresponding to the setting of the desired first machining increment and the number of such machining increments being equal to the selected number of machining increments. In the second mode of operation the microprocessor of the electronic depth controller acts upon the selected initial depth limit, the second desired machining increment, and the desired number of machining increments to provide signals to the microprocessor of the main control system whereby the main control system is effective to cause the electrodes initially to advance simultaneously in unison to the desired initial depth limit and thereafter repeatedly advance in successive machining increments each of which is equal to the setting of the second desired machining increment and the number of which is equal to the selected desired number of machining increments.
Breakthrough sensing means are also provided and are utilized in conjunction with both modes of operation. In the first mode of operation the breakthrough sensing means causes a fault indication to be given if breakthrough is detected anytime before completion of the full machining sequence consisting of the initial advance to the desired initial depth limit and completion of the full number of desired first machining increments. Accordingly, it may be perceived that the first mode of operation is preferrably utilized for the purpose of machining blind holes and hence a fault signal would be given if one or more of the holes had broken through.
The breakthrough sensing is utilized in conjunction with the second mode of operation for dual purposes. One purpose is to sense the intended breakthrough, meaning that all machined holes have broken through at the same time and that the cycle is therefore being completed. In this regard the breakthrough signal is utilized to arrest the machining cycle in such a way that only one additional desired second machining increment is allowed to occur subsequent to the machining increment during which breakthrough was sensed, even though the desired number of machining increments had not reached the full number which had been set on the control panel. This allows the operator the opportunity to check the workpiece to make sure that all holes have in fact been machined, even though the actual number of machining increments is less than the selected number set on the control panel. The second part of the dual purpose aspect of breakthrough sensing insofar as the second mode of operation is concerned is in conjunction with a fault indication. In this regard if breakthrough is sensed during advance of the electrodes to the initial depth limit, then a fault indication is given. Such a fault indication is desired because it is typically preferred that breakthrough occur during the progress of one of the desired second machining increments rather than during the initial advance of the electrodes to the initial depth limit. Accordingly, it can be perceived that the second mode of operation is intended to be selected where simultaneous breakthrough of multiple holes is desired.
A further aspect of the precision electronic depth controller involves a refeeding of the electrodes on the electrode holder at the conclusion of the arrival of the electrodes at the desired initial depth limit as well as a refeeding at the conclusion of each successive machining increment. The refeeding of the electrodes is for the purpose of maintaining a predetermined relationship between the tips of all the electrodes. One of the problems involved in multiple electrode machining is the possibility of uneven tip wear. Where multiple blind holes or multiple through-holes are being machined by a multitude of electrodes on a common holder, it is necessary for the electrode tips to be maintained in predetermined relationship so that the desired relationship between the respective depths of the blind holes in the first case, and the simultaneous breakthrough of the through-holes in the second case, always are obtained. Hence the electronic depth controller of the present invention provides for electrode refeeding, or redressing, to occur at the end of the initial advance to the desired initial depth limit as well at the end of each succeeding machining increment. The refeeding is accomplished by entirely conventional procedures which need not be described herein in detail. Preferably the electrodes are retracted from the workpiece to a location where the actual refeeding is accomplished.
A further advantage of the present invention is that the electronic depth controller is a unit which can be conveniently operatively associated with conventional EDM apparatus. Furthermore it possesses the capability for providing depth control of multiple EDM stations, although the present detailed disclosure of the preferred embodiment which hereinafter follows describes simply the operative association with a single EDM station.
The electronic depth controller of the present invention has the selectable input devices mounted on a common control panel and this control panel is particularly convenient for the machine operator and is conducive to having the operator quickly and accurately set depth parameters. The invention contemplates that a basic setting for a workpiece will be determined and set on the control panel switches. The operator may observe the work in progress and if a fault occurs adjustment in settings can be made as required. Once a machining cycle has been completed, the operator can observe the workpiece and determine if additional machining is required. If such turns out to be the case the depth parameters may be conveniently and quickly reset by the operator so that the job can be expediently and efficiently completed. Hence, where there are variations from workpiece to workpiece, such as in dimension, hardness, etc. the invention is particularly well suited for rapidly processing workpieces which have such differences. Accordingly, the invention in addition to improving the accuracy and precision of machining operations can increase the number of workpieces which can be processed.
It is also desirable for the control panel to contain a numerical readout display of the accumulated depth. This enables the operator to know the position of the electrode tips at all times during machining operations.
The foregoing features, advantages, and benefits of the invention, along with additional ones, will be seen in the ensuing description and claims which should be considered in conjunction with the accompanying drawings. The drawings disclose a preferred embodiment of the invention in accordance with the best mode contemplated at the present time in carrying out the invention.